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SPECIALTY PLASTICSPolyimide (PI)

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Polyimide (PI)

Imide group

C

O

O

N

C

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General Description

Most important group of highly heat resistant plastics, often referred to as "exotics".

–the polyamide imides, –the polyesters and –the polyetherimides as well as –imides with heterocyclic groups

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Manufacture by polycondensation or polyaddition.

Polycondensates are usable as thermosets or thermoplastics. However this does not mean that they are moldable as a melt.

Polyaddition products are useful only as thermosets.

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The Du Pont polyimides, such as Vespel (polycondensation polymer), Pyroline laminates and Kapton films have melting points which are far above the decomposition temperature.

Torlon (Amoco) and Polyimid 2080 (Upjohn) are thermoplastic polyimides.

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Some polyimides demand special processing techniques, so they are available only as semi-finished products.

The semi-finished products are manufactured by the powder sinter technique.

There are also polyimide grades which can be processed by compression molding but at very high temperatures and further annealing is required for best results.

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Polyimide (PI)

Some polyimide grades can be injection molded.

The melt temperature is 350 °C and the injection pressures are very

high. The cycle times are long because of

the large temperature difference during cooling.

Injection moldable polyimides can also be extruded.

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Polyimide (PI)

Multiple laminates are manufactured in heated presses, using the vacuum bag method, or in low-

pressure autoclaves. Filament winding is also common.

This method involves a heat-, cure- and cool-program. For condensation resins, pauses must be interspersed for escape of the volatile by-products so as to achieve

a product essentially free of voids. [The volatile side-products of polycondensation reactions are aromatic acids, aromatic nitrogen

compounds and solvent vapor.]

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Polyimide (PI)

• The polyimides can also be applied by spraying.

• Polyimide on aluminum gives glossy, heat-resistant coatings which are resistant to solvents and prevent

stain formation.

• There are many adhesives for bonding polyimide molded articles to

each other or to other materials.

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Polyimide (PI)

General DescriptionOften the advantages of polyimides cannot be exploited as only a limited selection of

moldings & semi-finisheds can be manufactured.

Up to now it has not been usual to process molding compounds or prepregs in plastics

processing plant.

The raw material manufacturers prefer to carry out the complicated processing themselves and produce moldings to clients' specifications. Some polymer manufacturers thus also offer finished

parts commercially

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Polyimide (PI)

Structure and General PropertiesAs the aromatic or heterocyclic ring

compounds in the chains of the macromolecule are closely pressed

together, the properties are- high strength over a wide temperature

range (- 240 to + 370°C),

- high stiffness and hardness,- high thermal stability,

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Polyimide (PI)

Structure and General Properties

- favorable friction and abrasion performance,

- good electrical properties,- high resistance to high energy

radiation,- highly flame resistant,

- low outgassing under high vacuum,- good resistance to chemicals and

water.

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Polyimide (PI)

Additives

Functional additives and reinforcements are of importance:

• Molybdenum disulfide (MoS2), graphite and PTFE for bearing material.

• Glass-, Carbon- and aramide-fibers are used as reinforcements.

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Polyimide (PI)

AvailabilityA wide variety of imide-based thermosets

and thermoplastics with functional additives and reinforcements are

commercially available:• As resins and compounds in the form of

dry powders or dissolved in solvents • Compressed and machined moldings • Prepregs in the B-state, honeycombs,

films, laminates, foams, coating materials, wire enamel and adhesives.

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Polyimide (PI)

AvailabilityThe polyimides - Vespel moldings and

Kapton films are products of the firm Du Pont de Nemours (US) and can only be obtained as finished precision moldings

and films from Du Pont.

There is a limited availability of semi-finisheds in the form of rods, tubes,

profiles, sheets, rings and disks.

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Polyimide (PI)

Resistance to Chemicals• Polyimide is resistant to solvents, fuels, grease, oils, detergents, dilute

acids and alkalis

• But not resistant to strong acids and alkalis or oxidants.

• It is not resistant to long-term exposure to boiling water and steam although the decrease in strength is

reversible. • It is resistant to stress crack formation

except in alkaline media.

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Polyimide (PI)

Weathering Resistance• Molded articles lose strength and

elongation at break after long exposure outdoors and should

therefore not be used. However they resist mold.

Resistance to High Energy Radiation• Molded items of PI are resistant up to

an absorbed dose of 107 J kg-l, higher than most other plastics.

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Polyimide (PI)Water Absorption

Moisture content of polyimide at equilibrium under normal environmental

conditions (23°C of, 50% RH) is 1.25% - the dimensional changes caused by alteration

of the moisture content of the surroundings is small

After 1000 hrs - 0.11 % in MD & 0.06% in TD

After 10000 hours - 0.28 in MD & 0.16% in TD

Only very narrow tolerances and extreme conditions preclude the use of PI.

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Polyimide (PI)

Machining

Machining is performed with standard carbide-tipped steel metalworking tools.

The materials must remain cool enough to be touched with the bare hand.

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Polyimide (PI)

Joining• The preferred method of joining is by

adhesion. • Polyimide molded items can be bonded

to each other and to other materials. • Choice of adhesive is made according to

the operating temperature. • For service temperatures below 150°C

Epoxide resins are suitable. Phenolic resin adhesives are somewhat more

heat resistant.

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Polyimide (PI)Applications

In the form of moldings and machined items, PI are used for piston rings, valve

seats, bearings, seals, welding-torch handle grips, jet engine accessories, electronic connectors, coil cores, covers for meters

and automobile accessories.

PIs are also used as valve stems in valves. High thermal stability and heat resistance

are the reasons for application in connection with soldering and welding.

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Polyimide (PI)

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Polyimide (PI)Applications

The largest and most important area of application for PI items up to now-is in the engines, for example

in the Boeing 747 for seals in the different compressor stages.

More recently because of the favorable friction and wear characteristics and lubricant-free operation, polyimide molded articles have been used in office

machinery and computers as bearings, rollers, slide and guide rails.

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Polyimide (PI)

• Kapton films are successfully used in a wide range of applications from -269°C to +400°C. These films are flame resistant, do not melt, are resistant to solvents and to high-energy radiation.

• PI films are used for insulating washers, cable and wire sheathing as well as printed circuit substrates.

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Polyimide (PI)

• Films coated on one or both sides with FEP can be heat sealed and act as a moisture barrier. Films coated by vaporizing aluminium protect space ships against the rays of the sun and the effect of heat on the power plant. Space suits are also insulated with such films.

•Polyimide films serve as sound insulation at high operating temperatures.

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Polyimide (PI)

MANUFACTURERS

Vespel - Solvay Advanced Polymers.

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Polyimide (PI)Further Reading

1. BRYDSON J.A, Plastics Material, Butterworth Heinemann, oxford, New

Delhi (2005)2. DOMININGHAUS H., Plastics for

Engineering, Hanser publishers, Munich, New York (1998)

3. CHARLESS A . HARPER, Modern Plastics Hand Book McGraw –Hill, New York

(1999)4. MARGOLIS J. M., Engineering

Thermoplastics, Marcel-Dekker, New York (1985)

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Table- I Guide values of properties of PIProperties Units PI PI (Glass Filled)

Density g/cm3 1.43 1.75

Water absorption (23°C, 24h) % 25- 50

Mechanical

Tensile strength N/mm2 75-100

Impact strength (Izod) J/m 3.8-7.6

Rockwell hardness scale 48-58 E

Thermal

Service temperaturein air without mechanical loading

short-term 400 400

long-term 260 260

Glass transition temperature °C 360

Heat deflection temperature °C 280-360 18 – 38

HDT, ISO Method A °C 0.35 – 0.41

Coefficient of linear expansion between 0 and 50 °C mm/mm/°C 50-63

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Table- I Guide values of properties of PIProperties Units PI PI (Glass Filled)

Thermal conductivity W/mK 0.29-0.35

Electrical

Volume resistivity cm 1016-1017

Surface resistance 1015- 1016

Dielectric constant 3.41

Dissipation factor, tan 50 Hz 0.0011

Dielectric strength kV/mm 22

Tracking resistance class C >300 >250

Water absorption (24h) mg 0.32 0.41